WASHINGTON -- The first six B-2 Stealth bombers ordered by the Air Force have been so plagued by manufacturing defects, last-minute design changes and other assembly delays that contractors may be unable to build the planes as fast and efficiently as desired by the Bush administration, a congressional investigation shows.

Although the B-2 contracting team -- led by Northrop Corp. -- has improved the manufacturing process with each new plane, the results continue to fall far short of goals set by company and Air Force officials to reduce the number of defects and delays, investigators said.

The first of the revolutionary radar-evading bombers, which rolled out in 1988, had 141,000 defects during its final assembly, investigators said after examining Northrop's records. The second B-2, expected to incur no more than 80,000 defects at the same stage of production, had 131,000 flaws, while the more-sophisticated third model, completed in June, had 117,600 -- many more than the 101,000 defects anticipated by the company.

Current plans for production of a seventh plane include a Northrop estimate that 71,200 defects will be found during the final assembly process.

"They've got to be making better progress, or it's going to cost a heck of a lot more to build those planes," one investigator said last week.

The review of the $65 billion program was done by the General Accounting Office, the investigative arm of Congress, at the request of Representative Les Aspin, D-Wis., chairman of the House Armed Services Committee.

Despite the GAO findings, which are being widely circulated on Capitol Hill, there were strong indications last week that the Stealth bomber would survive House and Senate negotiations over differences in the 1992 defense budget. The talks are expected to begin Sept. 11.

For its part, the Air Force Friday acknowledged a high number of defects in the bombers, but called them "inevitable" in a maturing design and manufacturing process, especially one that involves groundbreaking technological advances.

"The Air Force has never established a goal for an acceptable number of defects for the B-2 bomber," the statement said. "To us, the only acceptable number of defects is zero."

rTC For the second consecutive year, the Aspin committee and the full House have decided to stop B-2 production at the 15 aircraft already ordered, setting up another battle with the Senate and White House. The Senate voted Aug. 1 to spend $3.2 billion to build four more planes, approving a Bush administration strategy to improve efficiency and help trim overall costs by increasing the annual rate of production from two to seven planes per year by 1993.

President Bush has called for a fleet of 75 planes with a peak production rate of 16 aircraft per year by 1998.

Although the House tried to cancel the program last year, House and Senate negotiators worked out a one-year compromise to keep the B-2 alive. Now, B-2 critics believe the bomber program may survive yet again, this time with a compromise to build only two more planes.

There was mounting speculation last week that House negotiators might even consent to building three more B-2s in return for Senate agreement to stop production at 30 to 40 planes. House and Senate committee leaders were either unavailable for comment or refused to discuss their negotiating options.

Critics of the bat-winged plane, which they regard as too expensive and unnecessary, given the weakening of the Soviet Union, welcomed the GAO findings on the aircraft's recurring manufacturing defects. But the critics conceded that the effect on forthcoming budget negotiations may be overwhelmed by domestic politics.

"I've been saying for several weeks that facts don't matter," said Representative John Kasich, R-Ohio, a House Armed Services Committee member who has led the fight against the bomber, now costing $860 million per plane.

Manufacturing problems, no matter how extensive, as well as the likelihood that Soviet defense spending will take a nose dive, will take a back seat to 1992 election-year pressures and pork-barrel politics, Mr. Kasich complained.

He and other B-2 opponents suggested that several Democrats who opposed the Persian Gulf war have come out strongly for the Stealth bomber program to head off campaign attacks of being too soft on defense.

"Until the planning and manufacturing process becomes more reliable, there is a high risk that the contractors may not be able to achieve predicted efficiencies at planned, higher production rates," the GAO said.

Extensive defects found in the final assembly process, along with evidence of thousands of last-minute revisions to engineering drawings, show that "disciplined and rigorous production management programs are not fully in place," the GAO said.

An investigator said many manufacturing defects have been as minor as surface dents and paint scratches, but others include improperly drilled holes and the assembly of sections using wrong bolts and threaded fasteners.

Last year, more than 85,000 fasteners were reinspected by the contractors. More than 11,000 were found to have been used incorrectly, the GAO said.

The Air Force authored its own figures on the flaws. It said the first three bombers, which are undergoing flight testing, had 134,943, 123,984 and 136,134 defects, respectively, in the final assembly stage.

Between the third plane and the seventh, now in early stages of construction, "there has been a 38 percent reduction in defects across the program," the service said. "These figures continue to improve on subsequent vehicles."

Sixty percent of the defects found last year were "minor," and only 4 percent were actually serious enough to require parts to be scrapped, the Air Force said.